Sometimes, a sledge - hammer is the right tool ... :-)

Arfa Daily · 2010-10-21T01:05:18+00:00

Well, figuratively speaking, anyway !Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds flickered and flashed pathetically, before it settling with a couple of the output bar leds alight. Other than this, it did not a lot. Oddly, at the bottom of the two bar columns, are two more leds marked "DC" and "+15" and "-15". The "+15" was alight, but not the "-15".The power supply is a separate linear unit, screwed to the bottom of the desk, and connected to the main board by a ribbon cable. I was able to measure +15v and +5v at this connector, but no -15v, which seemed to go along with with what the diagnostic leds on the front panel were saying. However, with the power supply unplugged, -15v returned, at which point, my heart sank. You could measure a virtual dead short to ground on the -15v pin at the board end.It's a single board, so the next 20 minutes were spent removing knobs, screws and jack nuts to get the board out. It was double sided with thousands of tiny vias, and a mixture of surface mount and through hole technology, including many through hole SIL opamps. Shorts like this are very difficult to track down. It could be any decoupling capacitor anywhere on the board, or any IC. I tried all the usual stuff like using a low ohms meter to see if I could get close to the location, but the results were inconclusive. Nothing was running hot, as the linear regulator was in full overload foldback, so not supplying enough current into the short.So I decided it was make or break time, and got out my sledge - hammer in the form of a bench power supply. I disconnected the unit's own power supply, and set my bench one to -12v and a current limit of 1 amp, hooked it to the shorted rail, and settled back to wait for the smoke. There wasn't any, so I went round with a MK 1 finger to see if I could find a hot IC. I finally burnt my finger on a little surface mount 4880. Quicker than you could say "soldering iron" I had it whipped off the board. Then I...

Oct 21, 2010 24 Replies

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Wild_Bill 15 years ago

An instrument designed for this type of troubleshooting is a milli-ohm meter, with a 4-wire test probe set and an audio generator that changes the pitch of the tone (from the internal speaker), as the very small changes in resistance are detected while probing around the board. The speaker tone allows the operator to focus attention on the test points, and not need to read the digital display constantly.

ToneOhm is a common term associated withh these type of testers, although it may be a tradename used by Polar.

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navigate: Products to Toneohm 950

There have been many of the older, basic Polar models in the used instrument (and eBay) market.

The very basic model I bought (very inexpensivly on eBay) years ago wasn't a Polar, made in Britain instead, but I forget the manufacturer's name.

A basic instrument will have a couple of selectors for resolution and tone on/off.

Basic milliohm tester-type circuits can be found online.

-- Cheers, WB .............

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Meat Plow 15 years ago

You just like to argue. Don't you have something more constructive to do with your time?

Live Fast, Die Young and Leave a Pretty Corpse

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Jeffrey Angus 15 years ago

Yuppers, I bought one on eBay shortly after this came up the last time, Fun part it is.

Jeff

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Wild_Bill 15 years ago

I forgot to mench, I haven't used the Bob Parker - AnaTek Blue ESR meter enough yet, to see how effective it is at finding hard shorted faults on boards. Since I'm not involved in repairs daily, I might not need to actually try the Blue model for quite some time.

Although, I should have something around here that I could purposely put a short on, and try the Blue ESR meter.

The speaker with varying tone on the toneohm-type tester is definitely a handy way to test.

-- Cheers, WB .............

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JW 15 years ago

Yeah, I have a Toneohm 850A at work, but IMO they were too expensive for a single purpose such as that on my home bench. They're nice because you can use your ears to track and not have to keep looking up. The resolution is still only 100 micro ohms, though, same as the HP 3478A I mentioned.

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